Temporal lobe epilepsy (TLE) is the most common form of human epilepsy, often intractable to anticonvulsant therapy, whose treatment and prevention are critically dependent on understanding pathophysiological mechanisms in interlinked structures of the temporal lobe. The long-term objective of the proposed research is to identify and isolate components of the underlying epileptogenic circuits within two of these structures and to decipher mechanisms responsible for rendering them epileptic. The presubiculum (PrS) and parasubiculum (Par), relatively obscure structures lying at the interface between the hippocampus and entorhinal cortex (ERC), have been anatomically implicated in rendering neurons in the media entorhinal area (MEA) hyperexcitable during TLE. The proposed research is aimed at characterizing the PrS/Par physiologically and determining which of the recently identified neuronal populations in PrS/Par hyperexcitable become during TLE (Specific Aims 1 & 2). Anatomical studies have suggested that GABAergic projection neurons in PrS are functionally connected with inhibitory interneurons in LIII of MEA raising the possibility of PrS/Par-mediated control of local inhibitory circuits in MEA through disinhibition. However, neither inhibitory neurons of PrS/Par nor their projections to the MEA have been physiologically characterized. The proposed research will test this hypothesis by photouncaging glutamate focally to activate GABAergic neurons in PrS/Par while recording inhibitory synaptic responses from interneurons in MEA. Additionally, the alternative hypothesis that PrS is the source of persistent excitatory synapses to both excitatory and inhibitory neurons in LIII that perish during TLE will also be tested (Specific Aim 3). Without a systematic and deliberate effort to address these issues, it will be difficult to make any real progress in tracing the origins of TLE, determining its progress, and identifying what steps, if any, can be taken to intervene (identify location and means) and even to halt its progress. To this end electrophysiological, pharmacological and neuroanatomical techniques would be used in conjunction with a well-established animal model of TLE to characterize the PrS physiologically and test key hypotheses outlined above. Laser-scanning photouncaging of glutamate will be combined with CESOP and used in lieu of the labor-intensive paired recordings for a rapid, near-complete assessment of functional synaptic connectivity between PrS/Par and MEA as well as to measure the extent of synaptic reorganization that occurs in these structures during TLE.